Set up GitHub Actions CI pipeline for webifycms/domain

[Shifrin]

Goal

Automated quality checks on every push and pull request.

Pipeline steps

on: [push, pull_request]
jobs:
  quality:
    - Checkout
    - Setup PHP 8.1, 8.2, 8.3 (matrix)
    - composer install
    - composer code-sniff     (php-cs-fixer dry-run)
    - composer code-analyse   (phpstan level 8)
    - composer test            (phpunit)

Acceptance criteria

• Matrix runs against PHP 8.1, 8.2, 8.3
• PHPStan level 8 enforced
• Tests must pass on all PHP versions
• CS fixer runs in dry-run mode (fails on violations)
• Badge added to README